This invention relates generally to a muting circuit and more particularly to a circuit for muting a multichannel disc reproducing or playback apparatus, in the case where there is no reproduced angular modulated wave component.
In general, in a multichannel disc reproducing apparatus of a compatible, discrete 4-channel (CD-4) system, the reproduced signal is a superposed signal of a direct wave signal and an angular modulated wave signal. A 2-channel stereo record does not contain an angular modulated wave signal. If it is to be played in a reproducing apparatus capable of playing back a multichannel disc of this CD-4 system, it is necessary to cut off the demodulation system for angular modulated waves and to prevent noise from this demodulation system from being sent to the succeeding circuits.
For this purpose, heretofore, a muting circuit has been used for cutting off the demodulation system for angular modulated waves. A muting circuit for this purpose comprises, essentially, a muting control circuit for detecting the presence or absence of an angular modulated carrier wave and for generating a corresponding detection output signal, and a muting gate circuit for carrying out gating operation controlled by this detection output circuit.
While a muting control circuit in a muting circuit of known type is capable of positively detecting the presence or absence of an angular modulated wave component in a reproduced signal when such a component exists, it is incapable of positively detecting the absence of an angular modulated wave component in the reproduced signal. For example, sometimes an angular modulated wave does not exist in the reproduced signal but, a high frequency is in the audio frequency band, which does not have a carrier wave from the beginning. This frequency band is generated at the time of tracing of the 2-channel record disc, and it exists within the angular modulated carrier wave band. A muting control circuit of a known type inevitably detects this high-frequency component. In this case, therefore, this known muting circuit carries out an erroneous operation, and noise is generated in the reproduced sound.
Furthermore, in the case where an angular modulated wave within the reproduced signal is interrupted instantaneously and intermittently, a known muting circuit accomplishes muting in an intermittent manner. The resulting reproduced sound becomes discontinuous and unpleasant to the ear.